1. Field of the Invention
The present invention relates to agonists of muscarinic receptors. The present invention also provides compositions comprising such agonists, and methods therewith for treating muscarinic receptor mediated diseases. Particularly, the present invention is related to compounds that may be effective in treating pain, Alzheimer's disease, and/or schizophrenia.
2. Discussion of Relevant Technology
The neurotransmitter acetylcholine binds to two types of cholinergic receptors: the ionotropic family of nicotinic receptors and the metabotropic family of muscarinic receptors. Muscarinic receptors belong to the large superfamily of plasma membrane-bound G protein coupled receptors (GPCRs) and show a remarkably high degree of homology across species and receptor subtype. These M1-M5 muscarinic receptors are predominantly expressed within the parasympathetic nervous system which exerts excitatory and inhibitory control over the central and peripheral tissues and participate in a number of physiologic functions, including heart rate, arousal, cognition, sensory processing, and motor control.
Muscarinic agonists such as muscarine and pilocarpine, and antagonists, such as atropine have been known for over a century, but little progress has been made in the discovery of receptor subtype-selective compounds, thereby making it difficult to assign specific functions to the individual receptors. See, e.g., DeLapp, N. et al., “Therapeutic Opportunities for Muscarinic Receptors in the Central Nervous System,” J. Med. Chem., 43 (23), pp. 4333-4353 (2000); Hulme, E. C. et al., “Muscarinic Receptor Subtypes,” Ann. Rev. Pharmacol. Toxicol., 30, pp. 633-673 (1990); Caulfield, M. P. et al., “Muscarinic Receptors-Characterization, Coupling, and Function,” Pharmacol. Ther., 58, pp. 319-379 (1993); Caulfield, M. P. et al., International Union of Pharmacology. XVII. Classification of Muscarinic Acetylcholine Receptors,” Pharmacol. Rev., 50, pp. 279-290 (1998).
The Muscarinic family of receptors is the target of a large number of pharmacological agents used for various diseases, including leading drugs for COPD, asthma, urinary incontinence, glaucoma, schizophrenia, Alzheimer's (AchE inhibitors), and Pain.
For example, direct acting muscarinic receptor agonists have been shown to be antinociceptive in a variety of animal models of acute pain (Bartolini A., Ghelardini C., Fantetti L., Malcangio M., Malmberg-Aiello P., Giotti A. Role of muscarinic receptor subtypes in central antinociception. Br. J. Pharmacol. 105:77-82, 1992; Capone F., Aloisi A. M., Carli G., Sacerdote P., Pavone F. Oxotremorine-induced modifications of the behavioral and neuroendocrine responses to formalin pain in male rats. Brain Res. 830:292-300, 1999).
A few studies have examined the role of muscarinic receptor activation in chronic or neuropathic pain states. In these studies, the direct and indirect elevation of cholinergic tone was shown to ameliorate tactile allodynia after intrathecal administration in a spinal ligation model of neuropathic pain in rats and these effects again were reversed by muscarinic antagonists (Hwang J.-H., Hwang K.-S., Leem J.-K., Park P.-H., Han S.-M., Lee D.-M. The antiallodynic effects of intrathecal cholinesterase inhibitors in a rat model of neuropathic pain. Anesthesiology 90:492-494, 1999; Lee E. J., Sim J. Y, Park J. Y., Hwang J. H., Park P. H., Han S. M. Intrathecal carbachol and clonidine produce a synergistic antiallodynic effect in rats with a nerve ligation injury. Can J Anaesth 49:178-84, 2002). Thus, direct or indirect activation of muscarinic receptors has been shown to elicit both acute analgesic activity and to ameliorate neuropathic pain. Muscarinic agonists and ACHE-Is are not widely used clinically owing to their propensity to induced a plethora of adverse events when administered to humans. The undesirable side effects include excessive salivation and sweating, enhanced gastrointestinal motility, and bradycardia among other adverse events. These side effects are associated with the ubiquitous expression of the muscarinic family of receptors throughout the body.